(1) Field of the Invention
The present invention relates to a grain-oriented silicon steel sheet having a low iron loss, and a method of producing the steel sheet, and more particularly relates to a technique for lowering the iron loss of a grain-oriented silicon steel sheet by giving ununiformity to a coating film formed on the steel sheet surface so as to define and form, on the steel sheet surface, local (or localized) regions subjected to a tensile force different from that acting upon the remaining region of the steel sheet surface.
(2) Description of the Prior Art
Grain-oriented silicon steel sheets are mainly used in the iron cores of a transformer and other electric instruments, and are required to have excellent magnetic properties, particularly to have a low iron loss represented by W.sub.17/50.
In order to meet the requirement, it is necessary that the &lt;001&gt; orientation of secondary recrystallized grains in a steel sheet is highly aligned to the rolling direction of the steel sheet, and further that impurities and precipitates contained in the final product are decreased as possible. The iron loss value of grain-oriented silicon steel sheets produced so as to meet the requirements becomes lower year and year by laborous investigations and efforts, and recently a grain-oriented silicon steel sheet having a low iron loss value of W.sub.17/50 of 1.05 W/kg in a sheet thickness of 0.30 mm has been obtained.
However, since the energy crisis several years ago, electrical instruments having a lower iron loss are greatly in demand, and a grain-oriented silicon steel sheet having a lower iron loss has been demanded in order to use the steel sheet as an iron core material of the instruments.
As a means for lowering the iron loss of grain-oriented silicon steel sheet, there have generally been known metallurgical methods; for example, a method wherein the Si content is increased, a method wherein the thickness of a product steel sheet is made small, a method wherein the secondary recry stallization grains are made fine, a method wherein the content of impurities are decreased, a method wherein secondary recrystallization grains having a (110)[001] orientation are highly aligned, and the like. However, these means have been fully investigated, and the improvement of these means is very difficult, and even when the means are somewhat improved, the effect for lowering iron loss is very little.
A part from these means, Japanese Patent Application Publication No. 23,647/79 discloses a method, wherein secondary recrystallization-checking regions are formed on a steel sheet surface, thereby finely divided secondary recrystallization grains are made. However, this technique cannot control in a stable manner the size of the secondary recrystallization grain, and is not a practical method.
Japanese Patent Application Publication No. 5,968/83 disclose a technique, wherein slight strain is introduced into the surface of a secondarily recrystallized steel sheet by means of a ball-point pen-like small globe to subdivide the magnetic domain wall spacing, thereby the iron loss is lowered. Japanese Patent Application Publication No. 2,252/82 discloses a technic, wherein laser beams are irradiated to the surface of a final product steel sheet at an interval of several mm in a direction substantially perpendicular to the rolling direction to introduce high dislocation density regions into the surface layer of the steel sheet, thereby the magnetic domain wall spacing is subdivided to lower the iron loss. Japanese Patent Laid-open Application No. 188,810/82 discloses a technique, wherein slight strain is introduced into a steel sheet surface layer by means of an electric spark, thereby the magnetic domain wall spacing is subdivided to lower the iron loss. In these methods, slight plastic strain is introduced into the surface layer of a secondarily recrystallized steel sheet matrix, whereby the magnetic domain wall spacing is subdivided to lower the iron loss. These methods are practical methods, and are excellent in the effect for lowering the iron loss. However, the effect attained by the introduction of plastic strain into the steel sheet is lost by the heat treatments, such as stress-relief annealing and baking treatment of coating, which are carried out after punching, shearing, coiling and the like of the steel sheet. When it is intended to introduce very slight plastic strain into a steel sheet after a coating treatment, an insulating coating must be again applied to the steel sheet in order to maintain the insulating property. Also, additional steps, such as a strain-giving step and a recoating step, are required, resulting in a high production cost of grain-oriented silicon steel sheets. Japanese Patent Application Publication No. 17,757/78 discloses a technique for lowering magnetostriction of a grain-oriented silicon steel sheet by forming inorganic coating films having a stripe-shaped pattern or checkered pattern on both matrix surfaces of the steel sheet.
The object of the present invention is to provide a grain-oriented silicon steel sheet having excellent magnetic properties by subdividing the magnetic domain wall spacing based on a technical idea different from that of the above described prior art, which steel sheet can secure its excellent magnetic properties obtained by the subdivision of magnetic domain wall spacing, even after the stress-relief annealing at high temperatures.
The present invention is based on the discoveries that, when a forsterite film constituting a surface film of a grain-oriented silicon steel sheet has locally regions having a thickness different from that of the remaining regions in the forsterite film, the magnetic domain wall spacing can be very advantageously subdivided in the resulting grain-oriented silicon steel sheet; and that, when a tension-giving type insulating coating is applied onto the above described forsterite film locally having regions having a thickness different from that of the remaining regions in the film, the effect for subdividing the magnetic domain width can be more improved by their synergistic effect.
Hereinafter, in the specification, claims and drawings, the regions in a forsterite film, which have a thickness different from that of the remaining regions in the film, may be called as "forsterite film different-thickness regions", or merely called as "different-thickness regions".
The present invention is further based on the discoveries that, when a forsterite film constituting a surface film of a grain-oriented silicon steel sheet has locally filmless regions which do not coat the steel sheet surface, the magnetic domain width of the resulting grain-oriented silicon steel sheet can be very advantageously subdivided similarly to the presence of the forsterite film different-thickness regions; and that, when a tension-giving type insulating coating is applied onto the above described forsterite film locally having the filmless regions, the effect for subdividing magnetic domain width can be more improved by their synergistic effect.
Hereinafter, in the specification, claims, and drawings, the filmless regions in a forsterite film which do not coat the steel sheet surface may be called as "non-forsterite film regions" or merely called as "filmless regions".
In the production of grain-oriented silicon steel sheets, a cold rolled steel sheet having a final gauge is generally subjected to a decarburization annealing to remove harmful carbon. The decarburized steel sheet has a primary recrystallization texture containing an inhibitor, which forms a fine second phase dispersed in the interior of the steel sheet, and at the same time the surface layer of the steel sheet has a subscale structure consisting of the matrix and fine SiO.sub.2 grains dispersed in the matrix. After the decarburized and primary recrystallized sheet is applied on the surface with an annealing separator consisting mainly of MgO, the steel sheet is subjected to a secondary recrystallization and purification annealing (a final annealing) at a high temperature of about 1,200.degree. C. By this secondary recrystallization, the crystal grains in the steel sheet grow into coarse grains having a {110}&lt;001&gt; orientation. Moreover, by the high temperature purification, a part of inhibitors, such as S, Se, N, etc., which remain in the steel sheet, is removed from the steel sheet matrix.
Furthermore, in this purification, SiO.sub.2 in the subscale of the surface layer of the steel sheet and MgO in the annealing separator coated on the steel surface are reacted with each other according to the following equation: EQU 2MgO+SiO.sub.2 .fwdarw.Mg.sub.2 SiO.sub.4
to form a coating film consisting of a polycrystal of forsterite (Mg.sub.2 SiO.sub.4) on the surface layer of the steel sheet. In this case, unreacted excess MgO serves to prevent the fusing between the fellow steel sheets. After the final annealing, the unreacted annealing separator is removed from the steel sheet, and if necessary, an insulating coating is finally coated or a coil set is removed to obtain a product steel sheet.
The inventors have reinvestigated the role of forsterite film, and found that the film gives a tensile force to a steel sheet to subdivide the magnetic domain wall spacing and the subdivision effect of the magnetic domain wall spacing in the steel sheet varies finely depending upon positions. As the result, the inventors have reexamined carefully about the subdivision effect of the magnetic domain wall spacing in a steel sheet, and found that the above mentioned effect is remarkable in a place where the thickness of the forsterite film changes.